1. Field of the Invention
The present invention relates generally to a filter in which a noise component of a chrominance signal is reduced in a color picture signal field such as a video tape recorder field, a movie camera field, a video printer and the like, and particularly to a filter in which a noise component of a chrominance signal used for a video tape recorder according to standards of the National Television System Committee (NTSC) is reduced. Also, the present invention relates to a chrominance signal noise component method in which a noise component of a chrominance signal is reduced by using the filter.
2. Description of the Prior Art
A color picture signal has been recently reproducible in a high resolution by using a printing technology such as a sublimation transfer process while a hard copy technology such as a full-colored hard copy technology has been developed. A color signal reproduction in a printing technology has been advanced by improving a recording material and a picture processing technology, and the color signal reproduction of the printing technology has become the same level as that of a photography technology. Also, resolution in a printing technology has almost become the same level as that of a silver-salt photograph technology by using a high-definition picture signal according to a high definition television technology.
However, in a video printer in which a television signal used according to a current used technology is recorded and printed, the degree of a resolution for printing the television signal is limited because of a band-width compression of a picture signal according to standards of the National Television System Committee (NTSC). Therefore, in cases where the television signal according to the current used technology is printed, a resolution for printing the television signal is not sufficient as compared with that of the video printer. In particular, because a horizontal resolution according to the current used technology is only several tens picture lines for chrominance signals (or color difference signals) of a television picture, the horizontal resolution for a television image plane is very low, and a picture image having a great degree of color blur in a horizontal scanning line direction of the television is stored in the video printer.
2.1. Previously Proposed Art
To reduce a noise component of a chrominance signal such as color blur, a noise component reduction technology has been disclosed in U.S. Pat. Nos. 4,954,885, 5,225,899, 5,146,318 and 5,103,296. In detail, a color difference signal relating to a remarked pixel is processed to reduce a noise component by using signals relating to neighboring pixels adjacent to the remarked pixel when the color difference signal and a luminance signal included in a picture signal relating to the remarked pixel are separated. However, the noise component reduction is performed by merely using the signals relating to the neighboring pixels, the noise component reduction is locally performed. In particular, in cases where picture signals are hard-copied, because the recording density of the picture signals is high, a noise component reduction performed by using signals relating to wide-ranged pixels is required.
As a noise component reduction performed by using signals relating to wide-ranged pixels, video signal processing method and apparatus are disclosed in a Published Unexamined Japanese Patent Application (PUJPA) No. 158608/1993 (H5-153608). In the method and apparatus, a first chrominance signal relating to a remarked pixel and second chrominance signals relating to upper and lower pixels adjacent to the remarked pixel in a vertical direction of the television image plane are selected out, and an arithmetical means of the first chrominance signal and the second chrominance signals is calculated to obtain a smoothed chrominance signal relating to the remarked pixel. The reason that the second chrominance signals relating to the upper and lower pixels adjacent to the remarked pixel in the vertical direction are selected is because a vertical resolution in the television image plane is superior to the horizontal direction as is well known. Therefore, chrominance signals of a picture image are filtered in the vertical direction to smooth the chrominance signals without degrading the luminance of the picture image. In detail, the correlation among luminances of three pixels consecutively placed in the vertical direction of the television image plane is detected. In cases where a considerable variation of the luminances in the vertical direction is detected according to the correlation, it is judged that the remarked pixel is placed at a edge portion, and it is assumed that a variation of chrominance signals relating to the remarked pixel and the upper and lower pixels is large. Thereafter, the chrominance signals are slightly filtered to smooth the chrominance signals in a light degree. In contrast, in cases where a variation of the luminances in the vertical direction is slight, it is assumed that a variation of the chrominance signals is slight, and the chrominance signals are heavily filtered to smooth the chrominance signals in a heavy degree. The calculation for arithmetically averaging the chrominance signals is repeated several times to obtain a smoothed chrominance signal relating to the remarked pixel. Therefore, the chrominance signals are smoothed in the vertical direction by using the video signal processing apparatus simply manufactured, and noise components of the chrominance signals can be reduced.
2.2. Problems to be Solved by the Invention
However, because the calculation of the arithmetical mean for averaging the chrominance signals is repeated several times by using signals relating to wide-ranged pixels in the video signal processing method and apparatus disclosed in the PUJPA No. 158608/1998, there is a drawback that a luminance line memory for storing a plurality of luminance signals relating to all of pixels placed in the vertical direction and two chrominance line memories for respectively storing a plurality of chrominance signals placed relating to all of the pixels are required. Also, there is another drawback that a switching operation for switching addresses of the line memories in which luminance and chrominance signals are stored is required. Also, it takes a lot of time to read or write luminance and chrominance signals from/in the line memories. Therefore, to perform a picture image processing at a high speed, there is another drawback that the arithmetical mean of the chrominance signals cannot be sufficiently repeated.